Transmit diversity is one of the key technologies used in third generation (3G) wireless communications systems. By transmitting the signal through multiple transmit antennas to multiple receive antennas spatial diversity gains can be achieved to enhance the system capacity. Space-time transmit diversity (STTD) is an open loop technique in which the symbols are modulated using space-time block code, see Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE J. Select. Areas Commun., 16:1451-1468, 1998.
The STTD scheme is adopted by 3GPP due to its simple implementation and maximal diversity gains. Transmit adaptive array (TXAA) is a closed loop transmit diversity technique adopted by 3GPP. The mobile receiver feedbacks the estimated optimal transmit weights to the basestation. The base station uses this feedback information to adjust the power level of the transmitted signal so that the received power at the desired mobile receiver is maximized.
Simulation results show that the STTD is robust at higher velocities, while TXAA provides the biggest benefits at lower velocities, see Derryberry et al., “Transmit Diversity in 3G CDMA Systems,” IEEE Comm. Magazine, vol.40, no.4, pp. 68-75, April 2002.
A mixture of open and closed loop diversity technique could be used to combat both fast and slow fading. Recently, an adaptive STTD (ASTTD) scheme has been described, which combines STTD with adaptive transmit power allocation in order to improve the performances of the STTD systems, see Huawei “STTD with Adaptive Transmitted Power Allocation,” 3GPP TSG-R WG1 document, TSG-R1#26 R1-02-0711, Gyeongju, Korea May 13-16, 2002.